A row has broken out between unions and the government after ministers warned that next week's one-day strikes over public sector pension reforms could cost the economy up to £500m and lead to job losses.

With less then a week to go before the biggest walkout in decades, the ministers in charge of pensions negotiations, Francis Maude and Danny Alexander, said the strikes would impose a "significant hit to the economy at a very challenging time" as they urged public sector staff to go to defy their unions and turn up to work next Wednesday.

Unions hit back, saying the government was seeking to "scapegoat" public sector workers for poor economic growth and accused ministers of "fantasy economics".

The Treasury says the planned walkout by more than 2 million workers will lower output in the public and private sectors, and estimates that if everyone balloted goes on strike, it will cost the economy around £500m and put some people out of a job.

Maude, the minister for the Cabinet Office, said this was likely because there was a correlation between economic output and loss of employment.

"Exactly what that relationship is very hard to anticipate but if we lose a big chunk of output it is hard to see how that does not translate into fewer jobs," he said.

The £500m cost to the economy is the worst-case scenario. It was reached as a result of a modelling exercise and includes the cost to the public sector and the knock-on costs for the private sector, as well factoring in the cost of parents having to take time off work.

Ministers said the biggest impact will be caused by thousands of school closures, forcing parents to work from home, make childcare arrangements, or take their children to the office.

Brendan Barber, the TUC general secretary, rejected the government's estimate of the strike's costs.

He said: "While the strike on 30 November will obviously cause disruption, the figures suggested by ministers are fantasy economics.

"This is the clearest sign yet that next week's autumn statement will be a damp squib and the government is using the strike as yet another desperate excuse.

"Blaming the weather, the royal wedding and now scapegoating hardworking teachers, nurses and dinner ladies for the UK's economic woes is pretty poor from a government that has presided over record unemployment and the weakest economic recovery for a century."

The Unite union accused ministers of "plucking figures out of the air". Unite's assistant general secretary, Gail Cartmail, said: "The government have resorted to plucking figures out of the air to justify its refusal to negotiate constructively with the unions.

"If Danny Alexander and Francis Maude are so concerned about the impact of the strike on the economy, why are they refusing to make any attempt to negotiate further?

"We have resisted negotiating through the media because we prefer to sit across a table. The government has been a repeat offender, choosing to negotiate by megaphone and then offer too little, too late."

Alexander, the chief secretary to the Treasury, said that the last thing the economy needed at a time of financial turmoil and the eurozone crisis was a "self-inflicted wound" by unions taking strike action while pensions negotiations were ongoing. David Cameron suggested on Wednesday that firms should allow parents to bring their children to work where it is safe to do so, to minimise disruption to their working day caused by most schools closing as education unions – including the National Association of Head Teachers – take action over their pensions.

Alexander said he was considering taking his daughter to work with him at the Treasury for the day.

He said the timing of the strike was "bizarre" as talks were still being held, and would continue up to and beyond the walkout. But he said the government wanted discussions finalised by the end of the year, with legislation planned for the 2012/13 parliamentary session and the new pension arrangements implemented in 2015.

"There is no more money on the table. I have no better offer up my sleeve. We have set out what needs to be done. There is a real sense of purpose on our side to reach an agreement. The strikes are a distraction from that."

Maude repeated that the low turnouts in some union ballots strengthened the case being made for changes to increase the threshold of votes before action could go ahead.

However, two moderate civil service unions representing specialist and senior-ranking staff across a number of government departments are among those with the highest turnouts.

Contingency planning is under way in advance of next week's walkout. The Guardian revealed on Tuesday that civil servants across the government had been asked by the Home Office to work as border control officers during the strike.

Selected groups of government employees were contacted this week and asked if they were willing to walk through picket lines and check passports at airports and ports next Wednesday.

The tactic raises the prospect of Whitehall mandarins flicking through the documentation of airline passengers as they enter Britain.

It is the first time that the government has sought to recruit other members of the civil service to break a strike by immigration officials. A government source confirmed the plan but added that any recruits would be restricted to checking British passports and other "low-risk work".

A government spokesman could not say whether civil servants who worked could expect to receive any perks or how much training they would receive over the next week.

UK Border Agency staff in embassies across the world have already been offered taxpayer-funded flights to Britain if they are willing to work during the strike.

Emails seen by the Guardian show the government has asked immigration officials from India, South Africa and Russia to return to the UK to cover next Wednesday, when thousands of their colleagues plan to take industrial action.

Staff willing to return and cross a picket line would be allowed to extend their stay to spend time with their families. About 18,000 immigration officials could join the strike next week, a month before the deadline for a deal between the government and union leaders on pension reform. An estimated 3 million public sector workers, including teachers and probation officers, are expected to strike.

It is understood the head of human resources at the UK Border Agency, Joe Dugdale, phoned the Public and Commercial Services union (PCS) last week and asked if it would be willing to provide minimal cover on the day of the strike. The union refused.

Mark Serwotka, the general secretary of the PCS, said the government was in a "blind panic". He criticised ministers for failing to plan despite months of warning about the strikes, and insisted the government could still avert the action by calling unions in for urgent and meaningful talks.

Serwotka said the government was more interested in glossing over the past few weeks than trying to resolve the dispute.

The union leader seized on a point of order made in the Commons on Wednesday by Labour backbencher John McDonnell about the prime minister's claim that low and middle earners would get more from their pensions as a result of the reforms.

McDonnell said that a pension calculator published by the Cabinet Office that allows staff to see what their pension would be worth following an improved offer made by Alexander earlier in the month showed the examples cited by the chief secretary to the Treasury were "wrong" and that David Cameron's comments were "simply incorrect".

"The calculator showed that low and middle earners would get less at comparable retirement rates," said McDonnell. "The calculator has since being taken down from the website."

Serwotka said: "What that indicates is that rather than worry about the services on the day, rather than plan properly for 30 November, they have been engaging in a PR exercise putting out misleading information to try and force through damaging changes that are unfair. Less than a week before the strike, to suddenly turn round and act in a blind panic is completely irresponsible."

He added: "They could stop the strikes even now. Instead of scratching around trying to put untrained people on the frontline, they should say today they want urgent talks to try and resolve the dispute. They are going to do exactly the opposite of that."

Serwotka said the government had not moved "one single jot" on the "key questions" over the past six months.

"They are forcing people to work up to eight years longer, forcing people to pay thousands of pounds for less of a pension; it's completely unfair."